Misnamed parameter on requests

[cghan-oktana]

From Pardot's API documentation, the format parameter is used to set the API data format (xml or json), but currently ruby-pardot is setting it wrong:

def initialize email, password, user_key
  @email = email
  @password = password
  @user_key = user_key

  @format = "simple"
end

as simple is a value used for response format, in which case the output parameter should be used.

Also the documentation needs to be corrected: Output formats